Since people are confused regarding Intel 64-bit, here is a brief summary of what can run on which Intel processor.

To run a 64-bit application, you obviously need a 64-bit processor. All the machines that Apple currently ships have a 64-bit processor.

  • Only the ‘Intel Core Solo’ and ‘Intel Core Duo’ processors are 32-bit. On such machines, you won’t be able to run 64-bit apps.
  • The ‘Intel Core 2 Duo’ and ‘Xeon’ processors are 64-bit.

Useful link on Apple Support: How to tell if your Intel-based Mac has a 32-bit or 64-bit processor.

If you have a 64-bit processor (‘Intel Core 2 Duo’ or better), there are 3 levels of 64-bit applications:

  • to run a 64-bit faceless application: you need 10.4 and later.
  • to run a 64-bit application with UI (Cocoa): you need 10.5 and later.
  • to run the 64-bit kernel (and run 64-bit kernel extensions): you need 10.6 and later and a K64-capable machine (the EFI needs to be 64-bit). By default only the Xserve (2008 and later) will run the 64-bit kernel. On other K64-capable machines, you have to explicitely turn on the 64-bit kernel to use it.

Useful link on Apple Developer: Introduction to 64-Bit Transition Guide.